reverse_list.rb

class LinkedListNode
  attr_accessor :value, :next_node

  def initialize(value, next_node=nil)
    @value = value
    @next_node = next_node
  end
end

def reverse_list(list, previous=nil)
  # Base Case: We have reached the last node of the original list.
  # This is when we take that last node and point it to the previous node.
  # Then we return the list (which will now be reversed).
  if list.next_node.nil?
    list.next_node = previous
    return list

  # Other Case: We aren't yet at the end so let's keep reversing.
  # To keep reversing, we assign the current node value into previous,
  # and we reverse the rest of the list.
  #
  # NOTE: If we don't create a new previous node with an empty next_node and
  # instead just pass list as previous, we will get an infinite loop.
  else
    # return reverse_list(list.next_node, list) # <--- Infinite loop
    previous = LinkedListNode.new(list.value)
    return reverse_list(list.next_node, previous)
  end
end